FIG. 1 shows an illustration of a light emitting device 10. The light emitting device 10 comprises a plurality of LED cells 11 (A, B, C, C1, C2, C3) connecting in series by conducting metals 13 on a single substrate 15, wherein each LED cell 11 comprises a first semiconductor layer 17 on the substrate 15, a second semiconductor layer 19 on the first semiconductor layer 17, an active layer 47 (not shown in FIG. 1) arranged between the first semiconductor layer 17 and the second semiconductor layer 19, and a conducting metal 13 arranged on the second semiconductor layer 19. When one polarity of the AC input passes from conducting region α to conducting region β, the current flows through the LED cells 11 in the following order: A→C1→C2→C3→C→B. The largest potential difference of the LED cells 11 occurs between LED cells A and B. As shown in FIG. 1, the serially-connected LED array further comprises a serially-connected sub-array with four LED cells 11 (C1, C2, C3, C) intervening the terminal LED cells A and B in the series connection.
As shown in FIG. 1, LED A and LED B further comprise a first side (A1, B1) and a second side (A2, B2), respectively. The first sides (A1, B1) of LED A and LED B neighbor to the sub-array, and the second sides (A2, B2) of LED A and LED B neighbor to each other. Besides, a trench T is formed between LED A and LED B. Namely, the trench T is formed between the second sides of LED A and LED B.
Normally, the forward voltage for one LED cell 11 is about 3.5 volt, so the voltage difference between LED cells A and B should be about 3.5*6=21 volts under normal working situation. Because the distance between LED cells A and B is very short (about 10˜100 μm), the electric field strength (E=V/D, V=potential difference, D=distance) between LED cells A and B is high.
Besides, if there is suddenly a strong electrostatic field from the outside environment (such as from the human body or the working machine) injecting into the conducting region α, an ultra-high electrical voltage is further inputting to LED cell A, and causes the largest potential difference between LED cells A and B. When the value of the electric field strength reaches a certain value by the strong electrostatic field from the outside environment, the mediums (air, glue, or other dielectric materials) therebetween may be ionized, and parts of LED cells A and B within the electrical field strength are damaged (the damage region 12) by discharging, which is called the ESD (electrostatic discharge) damage. The SEM picture of the ESD damage situation is shown in FIG. 2, wherein the ordinary current flow 14 flows in the direction as the arrows indicated in the figure.